[SF] Don't crash if HWC returns an unknown config

When we call getActiveConfig() after HWC updates its modes
and before notifying us, we'll get an unknown configuration.
This change fixes a null pointer dereference in this scenario.

Bug: 158481338
Test: m surfaceflinger && flash
Change-Id: Ifa812628d238e4eb7873a163c834abc9f380bb0a